This application claims the benefit of U.S. application Ser. No. 61/892,676 filed on Oct. 18, 2013, the content of which is incorporated herein by reference in its entirety.
The subject disclosure relates generally to die-casting and in particular, to a wear ring for a die-casting piston, a die-casting piston incorporating the same, and a method of forming the same.
In the field of automotive manufacturing, structural components that historically have been fabricated of steel, such as engine cradles, are increasingly being replaced with aluminum alloy castings. Such castings are typically large, convoluted, and relatively thin, and are required to meet the high quality standards of automotive manufacturing. In order to meet these requirements, vacuum-assisted die-casting is typically used to produce such castings.
Vacuum-assisted die-casting machines comprise a piston, sometimes referred to as a “plunger”, that is advanced through a piston bore, sometimes referred to as a “shot sleeve”, to push a volume of liquid metal into a mold cavity. Vacuum is applied to the piston bore to assist the flow of the liquid metal therethrough. A replaceable wear ring is fitted onto the piston, and makes continuous contact with the inside of the piston bore along the full stroke of the piston for providing a seal for both the vacuum and liquid metal. The wear ring sits freely on a circumferential rib rearward of the front face of the piston tip, and is split to allow it to be installed onto the piston tip prior to use, and to be removed from the piston tip after use.
For example, FIG. 1 shows a portion of a prior art vacuum-assisted die-casting apparatus, which is generally indicated by reference numeral 20. Vacuum-assisted die-casting apparatus 20 comprises a piston that is moveable within a piston bore 22 for pushing a volume of liquid metal (not shown) into a die-casting mold cavity (not shown) to form a casting. In the example shown, the piston is positioned at its starting position of the stroke, which is rearward of a port 24 through which the volume of liquid metal is introduced into the piston bore 22.
The piston comprises a piston tip 30 mounted on a forward end of a piston stem (not shown). The piston tip 30 has a front face 32 that is configured to contact the volume of liquid metal introduced into the piston bore 22 via port 24. The piston tip 30 has a circumferential rib 34 formed on an outer surface thereof adjacent the front face 32, and an upper bore 36 positioned on a top surface of the piston tip 30 for receiving a removable retainer pin 38. The piston tip 30 has a wear ring 40 disposed on an outer surface thereof.
The wear ring 40 may be better seen in FIG. 2. The wear ring 40 consists of a body having a generally annular shape, and comprises an inner circumferential groove 42 that is shaped to receive the circumferential rib 34 of the piston tip 30. The wear ring 40 also comprises a gap 44 for enabling the wear ring 40 to be expanded as needed during installation onto, and removal from, the piston tip 30. The gap 44 is machined using conventional straight cutting, such that facing surfaces 46 of the wear ring 40 on opposite sides of the gap are parallel to the radial direction. The wear ring 40 further comprises a semi-cylindrical notch 48 machined therein at a diametrically opposite position from the gap 44, and which is shaped for accommodating the removable retainer pin 38.
During use, the wear ring 40 is installed onto the piston tip 30 by first inserting the retainer pin 38 into the upper bore 36, and then circumferentially expanding the wear ring 40 and fitting the inner circumferential groove 42 onto the circumferential rib 34 of the piston tip with the semi-cylindrical notch 48 aligned with the retainer pin 38. Once installed, the wear ring 40 is locked in rotational position relative to the piston tip 30, with the gap 44 being positioned at the underside of the piston tip 30, as may be seen in FIG. 1.
The piston having the piston tip 30 with the wear ring 40 installed thereon is then inserted into the piston bore 22 of the die-casting apparatus. As the wear ring 40 is sized such that its outer diameter is slightly greater than the inner diameter of the piston bore, insertion of the piston into the piston bore causes the wear ring 40 to compress circumeferentially. In response to the compression, the facing surfaces 46 of the wear ring 40 move into contact with each other. As a result of the straight cutting, the facing surfaces 46 of the wear ring 40 contact each other only partially, and along a contact line 45 as shown in FIG. 3. As will be understood, this partial contact provides a poor seal to any liquid metal exposed thereto.
Other die-casting pistons comprising wear rings have been described. For example, U.S. Pat. No. 5,048,592 to Mueller discloses a plunger for forcing molten aluminum or brass out of a casting cylinder of a die-casting machine. The plunger includes a cap that is screwed via an internal thread onto an external thread of a supporting body and is made of a material, in particular a copper alloy, which has a greater coefficient of thermal expansion than the material of the cylinder, in particular steel, and the material of the supporting body, in particular steel. In one embodiment, the cap has on its outer cover face a cylindrical extension with an outer annular web, which engages into a corresponding inner annular groove of a sealing ring. The sealing ring is split radially in a step shape.
U.S. Pat. No. 7,900,552 to Schivalocchi et al. discloses a piston for a cold chamber die-casting machine comprising a body and at least one sealing band mounted around the body. The body and the band are provided with coupling means for obtaining both an angular locking and an axial locking of the band to the piston body.
U.S. Pat. No. 8,136,574 to Müller et al. discloses a multi-piece piston for fixing to a high pressure side end of a piston rod running axially in a casting cylinder of a cold chamber casting machine. The piston comprises a piston crown forming a piston front face on the high pressure side and a piston body in the form of a bush connected to the piston crown on the low pressure side. Complementary bayonet locking means are provided for axial fixing of the piston to the end of the piston rod, on the piston crown and the end. In one embodiment, the piston carries slotted wearing rings on its outer circumference. The slotted wearing rings have slot edges forming a plurality of steps, with circumferentially running step surfaces bearing against one another.
It is an object at least to provide a novel wear ring for a die-casting piston, a die-casting piston incorporating the same, and a method of forming the same.